This was my first season with a pool and my first closing. The pool came with the house -- it's about 11 x 24, in-ground, vinyl with a 24 sq. ft. DE filter and a 3/4 hp pump. I calculated the volume to be about 12,500 gal. When I started the pool in the spring, I had trouble getting the filter to work. A pool service person determined that the previous owner used a cellulose-based DE substitute. I added standard DE which causes all sorts of problems. The service person said he washed the grids which seemed to fix the problem.
As part of my continuing education, I decided to take apart the DE filter and clean the grids. Somewhere I read that doing this once a year is a Good Idea™ -- recommended but not required. I got the filter apart with only minimal problems. What greeted me was a total surprise.
Even after a thorough backwashing, everything was caked with blue DE and what looked like sand. Over the swimming season I used nearly 25 lbs of DE and it was most definitely white when added. I estimate there was ~5 lbs of DE coating everything (~20% of what I added over the season) including several 10's of sq. inches which were at least a 1/2" thick. I hosed off the grids and used shop vac to clean the inside of the housing. I'm planning to have the grids acid washed over the winter.
First, is leaving 20% of the DE in the filter typical and normal? If so, washing the grids each year seems required. Also, why would the DE turn blue? I assume the sand was blow into the pool and collected in the filter when I vacuumed.
Any thoughts or comments would be appreciated.